1. Field of the Invention
The present invention relates generally to the characterization of the motion of particles suspended in a liquid medium and, more particularly, to an improved device and a method developed for characterizing the motion of sperm, bacteria, particles suspended in flowing fluids, Brownian motion and the like.
2. The Prior Art
The characterization of the motion of particles suspended in a liquid medium is of particular significance in fertility analysis. The term "characterization" as used in this specification and in the appended claims is intended to define a procedure involving analyzing and determining the motion of particles in a liquid medium. Motility analysis as regards fertility includes the determination of sperm motility and mean sperm velocity. The term "sperm motility" is intended to to be defined as the fraction of sperm moving among all the sperms in a given specimen sample. The term "progressive motility" is intended to define the fraction of sperm moving in an approximately constant direction. The term "progressivity" or "linearity" is the ratio between the distance travelled and the track length.
Motility analysis is undertaken regularly for animals, such as horses, in particular race horses, and prime bulls, in order to establish and to keep a permanent running record of the quality of their semen, hence their breeding potential. Motility analysis also represents an important segment in diagnosing certain reproductive problems in the human male.
For the most part, sperm motility and mean sperm velocity are simply estimated by visual examination of a drop of semen on a slide. The results of such visual examinations vary widely, often by as much as 40%, from one observer to another. Further, one cannot estimate, purely on a visual examination, linearity or velocity distribution functions. In order to determine such linearity or velocity distribution functions, a method of multiple exposure time-lapse photography has been developed. This method is tedious and time consuming in that it requires the manual counting of the sperm tracks, followed by manual derivation of the distributions of linearity and velocity. In order to speed up this manual method, a computerized version thereof has been developed which allows for the calculation of the distribution functions, but only after the sperm tracks first have been manually outlined by using an interactive indicating device such as a light pen or a "mouse". A further improved version employs a microscope attached to a computer, video recorder and other peripheral items. This improved version is designed to analyze a drop of semen in a special cell, called the Makler cell. The Makler cell is such that it maintains an exact narrow spacing, usually 0.01 mm, between its opposed walls. Such an exact narrow spacing is required so as to provide a sharp focus for the microscope image, to reduce the visual density and thus enable the computation of sperm density. The narrow spacing of the Makler cell, however, constricts the motion of the sperm tails. Sperm tails are believed to require 0.02 to 0.10 mm diameter about their axes, depending on species, in order to operate and move freely, i.e., without constriction. Consequently, a system employing the narrow Makler-type cell spacing adversely affects the very quantities, e.g., motility, velocity and linearity, that it is designed to measure. Such system also makes it very difficult to measure the motion of sperm in a diluent since sperm density is reduced to such a degree that obtaining statistically significant numbers of sperm requires a long time, and the superposition of many successive fields. The measurement of sperm motion in a diluent is frequently required in motility analysis. For example, in in vitro fertilization experiments, the semen may be diluted by a factor of 100 to 500. Further, the Makler cell is expensive to make and, in use, is difficult to maintain at an exact, controllable temperature.
In said prior pending application Ser. No. 897,036 filed Aug. 15, 1986, we have disclosed a motility scanner in which only light scattered by and from the specimen is able to reach the optical system. More specifically, the design of the prior motility scanner is such as to assure that the imaging lens thereof receives no directly transmitted light. As a result, the image formed appears bright on a dark background, a dark field image. Two significant advantages derive therefrom. First, analysis of sperm motion is facilitated since only the important objects, the sperms, appear bright and the microprocessor system is easily programmed to consider only bright objects. Second, a considerable depth of focus can be obtained, allowing thereby the use of specimen holding chambers of a size in which the motion of the sperm cells is not constrained by the presence of walls. These advantages, albeit remaining significant, are nevertheless obtained at the cost of poorer image resolution. Consequently, sperm cell heads appear as extended spots, and the sperm cell tails are visible only, if at all, in rare circumstances. As a result, it is impossible to tell what portion of the non-moving objects represents detritus or non-motile sperm cells. In accordance with the present invention, the presence of a visible sperm cell tail resolves this problem. A sharper image resolution also causes the sperm cell heads to appear reduced in size of the total area covered by the sperm cell heads. This reduced area, in turn, decreases the probability of sperm cell head collisions, since the apparent cross-section is smaller. The proportion of tracking errors at a given sperm cell concentration is markedly reduced thereby. Further, accurate analysis at higher values of sperm cell concentrations also is now possible.